I seem to be partial to Azusa for some reason. I would of picked Mio originally, but the second season (especially the later parts when Azusa starts really becoming like the rest of them) made me start leaning towards her as my favorite.
Hirasawa Ui, because she's so cute, mature, caring, and just crazy for her big sis. Kinda makes me wish I had a sister as sweet as her <3 Mio is a very close second, then Yui, and then Mugi, Ritsu, Nodoka, and Sawako (just because Sawako is at the bottom doesn't mean I don't like her. You bet I do :P)
One more thing: In case of people freaking out and asking me "but wheres AZUSA?!?!?!?" [sic], Azusa is actually the only character within the entire show that I don't like. She just strikes me as a cheap Mio knockoff :T